Tubular Composite Strut Having Internal Stiffening and Method for Making the Same

ABSTRACT

A system and method for forming a strut. A strut comprises a laminated composite tube having a substantially hollow interior and a pair of longitudinal stiffeners attached to opposite sides of the laminated composite tube.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 13/288,491, filed on Nov. 3, 2011, currentlyallowed, which has been published as 2013/0112309 on May 9, 2013, theentire disclosure of which is incorporated by reference herein.

BACKGROUND INFORMATION

1. Field

The present disclosure generally relates to composite columnarstructures, and deals more particularly with a composite tubular strutinternally stiffened to maximize the strut's strength-to-weight ratio.

2. Background

Columnar structures formed of composites are used in a variety ofapplications because of their favorable strength to weight ratio. Forexample, composite tubular struts may be used in the aerospace industryas a support or brace for transferring loads in either direction alongthe longitudinal axis of the strut, thus placing the strut in eithercompression or tension. Fittings on the ends of the strut provideadditional strength at the points of attachment of the strut to astructure.

Composite struts are known in which the end fittings, often fabricatedfrom metal, are attached to a tubular composite body by bonds ratherthan by fasteners. The tubular bodies have a substantially constantcross section and a relatively thick wall in order to meet design loadcriteria. The fittings may be attached to the ends of the tubular bodyby double step joints which may result in greater than desired peelforces being applied to inner and outer plies of the tube wall.Fabrication of these types of composite struts is both labor intensiveand time consuming because of the need for precise hand layup of plies,as well as the need for two autoclave cure cycles for separately curingthe inner and outer plies of the joint.

Accordingly, there is a need for a tubular composite strut that may bemore quickly fabricated and using less skilled hand labor. There is alsoa need for a composite strut as described above which has an improvedstrength-to-weight ratio.

SUMMARY

The disclosed embodiments provide a tubular composite strut and relatedfabrication method that optimize of the structural strength-to-ratio ofthe strut through the use of a tailored cross sectional tubeconfiguration that employs internal stiffeners co-cured with the tubebody, as well as the ability to tailor skin thickness. The strut isfabricated using an internal mandrel that allows layup of the internalstiffeners engineered to result in an idealized moment of inertia valueand maximize strut performance. The internal mandrel allows use of anautomatic fiber placement machine to layup plies of the tube body andthe stiffeners, which may reduce the need for an autoclave cure cycle toachieve a desired compaction. The tube body is bonded to the endfittings using a step joint with a single side bond. The internalstiffeners may be positioned and engineered to resist bending forcesdepending on the plane of loading from attachment, thereby allowing areduction of the thickness of the tube body wall.

According to one disclosed embodiment, a strut is provided comprising alaminated composite tubular body having a substantial hollow interior.At least one pair of stiffeners extends longitudinally through theinterior of and is attached to opposite sides of the tubular body. Thetube body may include end fittings adapted to attach the strut to astructure. In some examples, the fittings may include an opening havinga central axis and adapted to receive an elongate pin connecting thefitting to the structure. Each of the stiffeners is substantiallysymmetric about a plane extending perpendicular to the axis of the pin.Each of the fittings may include a pair of plurality of outer steps, andthe tubular body may include groups of plies respectively overlappingand bonded to the steps. The strut may further comprise a single shearjoint between each of the fittings and a corresponding end of thetubular body. Each of the stiffeners may be a laminated compositeco-cured with the tubular body.

According to another embodiment, a strut is provided for transferringloads in compression or tension. The strut comprises an elongate tubularbody having a wall formed of multiple laminated composite plies, and endfittings adapted to attach the tubular body to a structure. The tubularbody may be attached to the structure using a pin. The strut may furthercomprise a single shear bonded joint between each of the fittings andthe tubular body, and composite stiffeners within the tubular bodyattached to the wall for stiffening the body. Each of the end fittingstransfers loads to the tubular body about the longitudinal axis of thecorresponding pin. The composite stiffeners include at least a firstpair of opposing stiffeners aligned along an axis substantiallyperpendicular to the pin axes.

According to still another embodiment, a method is provided for making astrut. The method comprises locating composite stiffener layups incavities of an elongate mandrel, and forming a tubular composite body bylaying up composite plies on the mandrel overlying the stiffener layups.In some examples, fittings may be placed onto the mandrel prior tolocating composite stiffener layups in the cavities. In these examples,the composite plies of the tubular composite body may also overlay thefittings. The method further comprises cocuring the tubular body and thestiffeners and removing the mandrel. In some examples, the method mayfurther comprise fabricating an elongate mandrel, including formingcavities in the mandrel along its length. Forming the mandrel mayinclude forming an axle on each end of the mandrel, and placing thefittings on the mandrel is performed by placing the fittings on theaxles. The stiffeners may be formed offline and placed into the mandrelas a completed part layup. In other words, locating the stiffener layupsin the mandrel cavities may include laying up and forming thestiffeners, and placing the formed stiffener layups in the cavities. Inother examples, locating the stiffener layups in the mandrel cavitiesmay include using an automatic fiber placement machine to lay down andcompact strips of composite tape within the cavities. Forming thetubular body may be performed using an automatic fiber placement machineto lay down and compact strips of composite tape on the surface of themandrel. Forming the tubular composite body may include forming a singleshear joint between the tubular body and each of the fittings. Removingthe mandrel may include dissolving the mandrel with a liquid. The methodmay further comprise inserting stiffener mandrels within the stiffenerlayups, and removing the stiffener mandrels after the tubular body andthe stiffeners have been co-cured.

The features, functions, and advantages can be achieved independently invarious embodiments of the present disclosure or may be combined in yetother embodiments in which further details can be seen with reference tothe following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the advantageousembodiments are set forth in the appended claims. The advantageousembodiments, however, as well as a preferred mode of use, furtherobjectives and advantages thereof, will best be understood by referenceto the following detailed description of an advantageous embodiment ofthe present disclosure when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is an illustration of a perspective view of a composite strutaccording to the disclosed embodiments.

FIG. 2 is an illustration of a top view of one of the end fittings ofthe strut shown in FIG. 1.

FIG. 2A is an illustration of a side view in the direction shown as2A-2A in FIG. 2;

FIG. 3 is an illustration of a cross sectional view taken along the line3-3 in FIG. 1.

FIG. 4 is an illustration of a sectional view taken along the line 4-4in FIG. 3.

FIGS. 4A-4C are illustrations similar to FIG. 4A but respectivelyshowing alternate embodiments of the composite strut.

FIG. 5 is an illustration of a perspective view of a mandrel used tofabricate the strut shown in FIG. 1.

FIG. 6 is an illustration of a top view of the mandrel shown in FIG. 5,with the strut end fittings having been attached to the mandrel axles.

FIG. 7 is an illustration of a sectional view taken along the line 7-7in FIG. 6.

FIG. 8 is an illustration of a sectional view taken along the line 8-8in FIG. 7.

FIG. 9 is an illustration of a perspective view of a stiffener layup.

FIG. 10 is an illustration of a cross sectional, end view showing thestiffener shown in FIG. 9 being placed in one of the mandrel cavities.

FIG. 11 is an illustration similar to FIG. 10 but showing the stiffenerhaving been fully installed in the cavity and a mandrel having beeninstalled within the stiffener.

FIG. 12 is an illustration similar to FIG. 10, but showing an alternateembodiment of the stiffener being placed in a modified form of one ofthe mandrel cavities.

FIG. 13 is an illustration similar to FIG. 12 but showing the stiffenerhaving been fully installed in the cavity and a mandrel along withradius fillers having been installed.

FIG. 14 is an illustration similar to FIG. 13, but showing strips ofcomposite tape being laid up over the mandrel, covering the stiffenerand stiffener mandrel.

FIG. 15 is an illustration is a perspective view of an automatic fiberplacement machine applying composite tape to the mandrel assembly shownin FIG. 6.

FIG. 16 is an illustration of a flow diagram of a method of making acomposite strut according to the disclosed embodiments.

FIG. 17 is an illustration of a flow diagram of aircraft production andservice methodology.

FIG. 18 is an illustration of a block diagram of an aircraft.

FIG. 19 is an illustration of a cross-sectional view of a compositestrut according to the disclosed embodiments.

FIG. 20 is an illustration of a cross-sectional view of a compositestrut according to the disclosed embodiments.

FIG. 21 is an illustration of a cross-sectional view of a compositestrut according to the disclosed embodiments.

FIG. 22 is an illustration of a cross-sectional view of a compositestrut according to the disclosed embodiments.

DETAILED DESCRIPTION

Referring first to FIG. 1, a columnar structural member in the form ofan elongate strut 20 comprises a generally cylindrical tube 22 and apair of end fittings 24 bonded on opposite ends of the tube 22. The tube22 may comprise, but is not limited to a composite material, such asmultiple laminated plies of a fiber reinforced polymer resin, includingbut not limited to carbon fiber reinforced epoxy. In the illustratedexample, the cross sectional shape of the tube body 22 is substantiallyround, however other cross sectional shapes are possible such as,without limitation, square, triangular, hexagonal and pentagonal shapes.

Each of the end fittings 24 may be, but is not limited to a metal suchas aluminum or titanium, or a composite. The end fittings 24 may befabricated by casting, machining or other common manufacturingtechniques. Where the end fittings 24 are formed of composite materials,they may include metallic inserts and/or metallic bushings (not shown).Each of the end fittings includes clevis 26 comprising a pair of spacedapart tabs 28 respectively having an opening aligned along an axis 27for receiving an attachment pin 32, without limitation shown ascylindrical, which connects the fitting 24 to a structure (not shown).While shown as a double tab 28 clevis 26, more or less than two tabs 28may be employed, depending on the application. The pins 32, along withthe end fittings 24, form pivotal connections between the strut 20 andthe structure (not shown) to which it is attached. Strut 20 may beemployed, for example and without limitation, as a brace between anaircraft engine (not shown), and an airframe (not shown). Depending uponthe application, the strut 20 may function to transfer axial loadsbi-directionally so that the strut 20 may be either placed in tension orcompression, or both in an alternating fashion, along its central axis29. The strut 20 may also experience limited torsional loading.

Referring now to FIGS. 2, 3 and 4, each of the fittings 24 includes acylindrical section 35 having an outer diameter D₁ (FIG. 2) that issubstantially equal to outer diameter D₂ (FIG. 4) of the tube body 22.The end fittings 24 also include a scalloped section 40 of length Lformed by a series of cylindrical steps 42 of progressively smallerdiameter. Increasing the length L appropriately for the applicationassists in dispersing the “stiffness” associated with the intersectionof a metal fitting 24 and with the composite tube 22. In other words,increasing the length L of the scalloped section 40 disperses the loadtransferred between the end fitting 24 and the tube 22 over a largerarea. The interior of each fitting 24 may be substantially hollow,defined by a longitudinally extending smooth bore 44 having a diameterD₃ that extends through the length of the fitting 24. In the illustratedembodiment, the diameter D₃ of the bore 44 is substantially equal to theinterior diameter D₄ of the tube 22 in order to maximize the contactarea, and thus the load transfer capability, between the fitting 24 andthe tube 22. However, in other embodiments the diameter D₃ of the bore44 may be different than the interior diameter D₄ of the tube 22. Asshown in FIG. 2, the tabs 28 are spaced apart at 38 to receive a tab 33on a structure (not shown) to which the strut is to be attached.Although not shown in FIG. 2, the attachment pin 32 (FIG. 1) passesthrough each of the tabs 28, 33.

Tube body 22 comprises a plurality of stepped groups 45 of compositeplies 48 which may be formed from prepreg fiber reinforced tape applied,for example and without limitation, by an automatic fiber placement(AFP) machine (not shown). However, the use of other types of compositematerials such as, without limitation, metallics and ceramics, and othertypes of fabrication techniques may be possible. The stepped ply groups45 are sequentially laid up over the steps 42 of the end fittings 24,beginning with ply layup on the innermost step 42 a, and ending with plylayup on the outermost step 42 b. The stepped ply groups 45 respectivelyoverlap the steps 42 to form a single stepped shear joint extendingalong the length L the scalloped section 40 of the fitting 24. In otherembodiments it may be possible to employ a scarf joint (not shown)between the tube 22 and the fitting 24, which comprises multiples scarfswith the same or varying slopes The wall 50 of the tube body 22 has athickness t (FIG. 4). In the illustrated embodiment, the stepped plygroups 45 form a female tube end in which the male fitting 24 isreceived, however in other embodiments the stepped ply groups 45 mayform a male end of the tube 22, and the steps 42 of the fitting 24 maybe formed on the interior diameter D₃ of the bore 44 which receives themale end of the tube 22.

Referring now particularly to FIGS. 3 and 4, in one embodiment, the tube22 may be internally stiffened by two pairs of elongate compositestiffeners 52, 54, respectively. The first pair of stiffeners 52 arelocated diametrically opposite each other (FIG. 4) on the interiorsurface 47 of the tube wall 22, and are generally symmetrically alignedalong axis 25 extending substantially orthogonal to the axis 27 of thepin 30. The second pair of stiffeners 54 are also located diametricallyopposite each other, and generally symmetrically aligned along the axis27 of the pin 30. In the illustrated example, each of the stiffeners 52,54 is substantially trapezoidal in cross sectional shape, however othercross sectional shapes are possible. Each of the stiffeners 52 has alarger cross sectional area than stiffeners 54 in order to provide thetube 22 with greater resistance to bending about the pin axes 27. In oneembodiment, one or both of the stiffeners 52, 54 has a cross sectionalshape that is substantially constant along the length of the stiffener52, 54. However, in other embodiments, the cross section of any of anyof the stiffeners 52, 54 may vary in shape or area, either linearly ornon-linearly along the length of the stiffener 52, 54. In someembodiments, the stiffeners 52, 54 may be bonded to the tube 22 afterthe tube 22 is cured.

While the embodiment shown in FIGS. 3 and 4 employs stiffeners 54located inside surface 47 the tube 22, other stiffening arrangements arepossible. For example, FIG. 4A illustrates composite stiffeners 52, 54that are located on the exterior surface 53 of the tube 22, while FIG.4B illustrates stiffeners 52, 54 that are respectively located on theexterior surface 53 and the interior surface 47 of the tube 22. FIG. 4Cillustrates still another embodiment in which stiffeners 54 located onboth the interior and exterior surfaces 47, 53 respectively of the tube22.

Attention is now directed to FIG. 5 which illustrates an internalmandrel 56 that may be used to fabricate the strut 20 previouslydescribed. The mandrel 56 includes a generally cylindrical body 57having a generally cylindrical outer mandrel surface 58. The mandrelbody 57 also includes four circumferentially spaced, longitudinalcavities 62 therein, each having a cross sectional shape substantiallycorresponding to the cross sectional shapes of one of the stiffeners 52,54. The mandrel 56 further comprises a pair of generally cylindricalaxles 64 on the opposite ends thereof which may formed integral with themandrel body 57. Each of the axles 64 has a diameter 66 substantiallycorresponding to the interior diameter D₃ of the bore 44 in the fittings24 (FIG. 3). A centrally located through-hole 68 extends axially throughthe body 57 for purposes of which will be discussed later in moredetail.

The mandrel 56 may be formed from a commercially available material thatmay be dissolved away when subjected to a suitable liquid such as waterat a later stage of the fabrication process. In one embodiment, asuitable powdered material is mixed with water and poured or injectedinto a mold (not shown) in order to form the features of the mandrel 56.Following molding, the mandrel is cured, dried using a suitable heatsource such as a convection oven and then sealed, as required.Alternatively, a water soluble mandrel 56 may be fabricated usingadditive manufacturing processes. In another embodiment, the mandrel 56may be fabricated from a block of material using suitable materialremovable processes. In still other embodiments, the mandrel 56 may beformed of a material that is incinerated when elevated to apredetermined temperature, or may comprise an inflatable bladder that isdeflated and removed from the tube body 22 after the fabrication processis complete. In still other embodiments, the mandrel 56 may bedisassembled and removed in pieces after the tube 22 is cured.

Referring now to FIGS. 6 and 7, after the mandrel 56 has beenfabricated, the end fittings 24 are sleeved over the axles 64. As can beseen in embodiment illustrated in FIG. 7, each of the axles 64substantially completely fills the diameter D₃ of the bore 44 (FIG. 3)of the corresponding end fitting 24. In other embodiments, the axle 64may extend only partially though the length L (FIG. 3) of the bore 43.

Referring now to FIGS. 8 and 9, in one embodiment, the stiffeners 52, 54may be laid up and formed to shape using separate tooling (not shown)using conventional layup and consolidation processes. Then, as shown inFIG. 10, the formed stiffener layup 52, 54 may be inserted into thecorresponding mandrel cavities 60, 62. Alternatively, however, automatedequipment such as an AFP machine may be employed to directly layup thestiffener 52 within the cavity 62 by laying down strips (not shown) offiber reinforced prepreg tape within the cavities 60, 62 and compactingthe strips against the mandrel 56. Referring to FIG. 11, after thestiffener layups 52, 54 have been located within the cavity 62, suitablestiffener mandrels 84, which may comprise either a dissolvable solidmaterial or inflatable bladders, are placed inside the stiffener 52 inorder to maintain the shape of the stiffener 52 and prevent it fromcollapsing during subsequent fabrication steps.

One or more of the stiffeners 52, 54 may include features that mayimprove the structural connection between the stiffener 52, 54 and thetube 22. For example, referring to FIG. 12, the stiffeners 52, 54 mayinclude integrated lateral flanges 61 that are respectively receivedwithin recesses 63 formed in the surface 58 of the mandrel 56, adjacentthe cavities 60, 62. As show in FIG. 13, when the stiffeners 52, 54 areplaced in the mandrel cavities 60, 62, the flanges 61 lie substantiallyflush with the mandrel surface 58. Radius filler 65 may be installedbetween the stiffener mandrel 84 and the flanges 61 to aid in preventingthe flanges 61 from collapsing inwardly during subsequent compaction andcuring phases. Next, as shown in FIG. 14, composite plies 48 forming theply groups 45 (FIG. 3) are laid up over the mandrel 56, covering theflanges 61 and the scalloped section 40 (FIG. 3) of the end fittings 24.During this layup process, the stiffener mandrels 84 support the plies48 and react the ply compaction forces applied in those applicationswhere an AFP machine is used to perform the layup process.

FIG. 15 illustrates the use of an AFP machine 74 to layup either thestiffeners 52, 54 and/or the plies 48 forming the wall 50 of the tube22. The AFP machine 74 may be any of several different suitable typeswhich include a composite tape applicator head 78 controlled by a robot76. Mandrel 56 and end fittings 24 are supported on suitable rotisserie70 which rotates 82 the mandrel 56 about a central axis 80. The plies 48may include unidirectional fiber reinforcement having varying fiberorientations according to a predetermined ply schedule, including butnot limited to 0, +30, +45, 90, −60, −45, and −30 degrees, or otherfiber orientations. As previously mentioned, other types of automatedlayup processes may be employed to layup the stiffeners 52, 54 and/orthe plies of the tube wall 50, including without limitation, filamentwinding and fabric layup.

FIG. 15 summarizes the overall steps of the method previously describedfor fabricating the strut 20. Beginning at 86, the end fittings 24 arefabricated using suitable casting, and molding and/or machiningtechniques. At 88, the mandrel 56 is formed which includes forming orassembling the mandrel body 57, cavities 60, 62 and axles 64. At 90, theend fittings 24 are installed on the axles 64 of the mandrel 56. At 92,stiffeners 52, 54 are laid up and formed using separate tooling.Alternately, the stiffeners 52, 54 may be directly laid up within thecavities 60, 62 using an AFP machine. In those embodiments where thestiffeners 52, 54 are individually laid up and formed in separatetooling, they are then located within the mandrel cavities 60, 62 atstep 94. At step 96, stiffener mandrels 84 are installed within thestiffener layups 52, 54 along with additional elements such as noodlesor other fillers, as required. At step 98, the plies 48 of the tube wall50 are laid up over the mandrel body 57, covering the stiffener layups52, 54 and the scalloped section 40 of the end fittings 24. At 100, avacuum bag assembly (not shown) is installed around the strut layup. At102, the strut layup is compacted and cured using an autoclave or otherprocesses which apply heat and pressure to the layup. Finally, at 104,the mandrels 56, 84 are removed. In the embodiment where the mandrels56, 84 are formed from a water soluble material, the water may beinjected into one end or both ends of the through hole 68, resulting inthe mandrels 56, 84 being dissolved and flushed out through the throughhole 68.

Embodiments of the disclosure may be employed, without limitation, inthe context of aircraft manufacturing and service method 106 as shown inFIG. 15 and an aircraft 108 as shown in FIG. 16. During pre-production,aircraft manufacturing and service method 106 may include specificationand design 110 of aircraft 108 in FIG. 16 and material procurement 112.

During production, component and subassembly manufacturing 114 andsystem integration 116 of aircraft 108 in FIG. 16 takes place.Thereafter, aircraft 108 in FIG. 16 may go through certification anddelivery 118 in order to be placed in service 120. While in service 120by a customer, aircraft 108 in FIG. 16 is scheduled for routinemaintenance and service 122, which may include modification,reconfiguration, refurbishment, and other maintenance or service.

Each of the processes of aircraft manufacturing and service method 106may be performed or carried out by a system integrator, a third party,and/or an operator. In these examples, the operator may be a customer.For the purposes of this description, a system integrator may include,without limitation, any number of aircraft manufacturers andmajor-system subcontractors; a third party may include, withoutlimitation, any number of vendors, subcontractors, and suppliers; and anoperator may be an airline, a leasing company, a military entity, aservice organization, and so on.

With reference now to FIG. 16, an illustration of an aircraft 108 isdepicted in which an advantageous embodiment may be implemented. In thisexample, aircraft 108 is produced by aircraft manufacturing and servicemethod 106 in FIG. 16 and may include airframe 124 with plurality ofsystems 126 and interior 128. The disclosed mandrel may be used tofabricate various structural components of the airframe 124, such asstringers. Examples of systems 126 include one or more of propulsionsystem 130, electrical system 132, hydraulic system 134, andenvironmental system 136. Any number of other systems may be included.Although an aircraft example is shown, different advantageousembodiments may be applied to other industries, such as the automotiveand marine industries.

Apparatuses and methods embodied herein may be employed during at leastone of the stages of aircraft manufacturing and service method 106 inFIG. 15. As used herein, the phrase “at least one of”, when used with alist of items, means that different combinations of one or more of thelisted items may be used and only one of each item in the list may beneeded. For example, “at least one of item A, item B, and item C” mayinclude, for example, without limitation, item A or item A and item B.This example also may include item A, item B, and item C or item B anditem C.

In one illustrative example, components or subassemblies produced incomponent and subassembly manufacturing 114 in FIG. 15 may be fabricatedor manufactured in a manner similar to components or subassembliesproduced while aircraft 108 is in service 120 in FIG. 15. As yet anotherexample, a number of apparatus embodiments, method embodiments, or acombination thereof may be utilized during production stages, such ascomponent and subassembly manufacturing 114 and system integration 116in FIG. 15. A number, when referring to items, means one or more items.For example, a number of apparatus embodiments is one or more apparatusembodiments. A number of apparatus embodiments, method embodiments, or acombination thereof may be utilized while aircraft 108 is in service 120and/or during maintenance and service 122 in FIG. 15. The use of anumber of the different advantageous embodiments may substantiallyexpedite the assembly of and/or reduce the cost of aircraft 108.

Turning now to FIGS. 19-22, several different cross-sectional views ofdifferent composite struts are depicted in accordance with illustrativeembodiments. The tube 22 may be internally stiffened by two pairs ofelongate composite stiffeners 52, 54, respectively. The first pair ofstiffeners 52 are located diametrically opposite each other on theinterior surface 47 of the tube wall 22, and are generally symmetricallyaligned along axis 25 extending substantially orthogonal to the axis 27of the pin 30 (not depicted). The second pair of stiffeners 54 are alsolocated diametrically opposite each other, and generally symmetricallyaligned along the axis 27 of the pin 30. In some illustrative examples,tube 22 may have a pair of stiffeners which are diametrically opposed toeach other but are not symmetrically aligned along either axis 25 oraxis 27. The elongate composite stiffeners 52, 54 may be formed of anumber of plies. As used herein, “a number of,” when used with referenceto items means one or more items. Thus, each of stiffeners 52, 54 may beformed of one or more plies. In one illustrative example, stiffeners 52,54 may each be formed of about 3 plies to about 6 plies. In otherillustrative examples, stiffeners 52, 54 may each be formed of more than6 plies. In yet other illustrative examples, stiffeners 52, 54 may eachbe formed of less than 6 plies.

In the illustrated examples, each of the stiffeners 52, 54 issubstantially trapezoidal in cross sectional shape, however other crosssectional shapes are possible. As shown in FIG. 19 and FIG. 21, each ofthe stiffeners 52 may have a larger cross sectional area than stiffeners54 in order to provide the tube 22 with greater resistance to bendingabout the pin axes 27. As shown in FIG. 20 and FIG. 22, each of thestiffeners 52 and stiffeners 54 may have substantially the same crosssectional area. As depicted in FIG. 21 and FIG. 22, the stiffeners 52,54 may include integrated lateral flanges 61. Integrated lateral flanges61 may provide a greater surface area for bonding the stiffeners 52, 54to the tube 22.

In one embodiment, one or both of the stiffeners 52, 54 in FIGS. 19-22may have a cross sectional shape that is substantially constant alongthe length of the stiffener 52, 54. However, in other embodiments, thecross section of any of any of the stiffeners 52, 54 may vary in shapeor area, either linearly or non-linearly along the length of thestiffener 52, 54. In some embodiments, the number of plies of any of thestiffeners 52, 54 may vary along the length of the stiffener 52, 54. Forexample, there may be ply drops or ply additions in any of thestiffeners 52, 54 along the length of the stiffener 52, 54 in someillustrative embodiments. In some embodiments, the stiffeners 52, 54 maybe bonded to the tube 22 after the tube 22 is cured. In some otherembodiments, the stiffeners 52, 54 may be bonded to the tube 22 atsubstantially the same time as when the tube 22 is cured.

The description of the different advantageous embodiments has beenpresented for purposes of illustration and description, and is notintended to be exhaustive or limited to the embodiments in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art. Further, different advantageousembodiments may provide different advantages as compared to otheradvantageous embodiments. The embodiment or embodiments selected arechosen and described in order to best explain the principles of theembodiments, the practical application, and to enable others of ordinaryskill in the art to understand the disclosure for various embodimentswith various modifications as are suited to the particular usecontemplated.

What is claimed is:
 1. A strut, comprising: a laminated composite tubehaving a substantially hollow interior; and a pair of longitudinalstiffeners attached to opposite sides of the laminated composite tube.2. The strut of claim 1, wherein the pair of longitudinal stiffeners islocated within the substantially hollow interior of the laminatedcomposite tube.
 3. The strut of claim 1, further comprising a fitting onat least one end of the laminated composite tube that attaches to astructure.
 4. The strut of claim 3, wherein each fitting includes anopening having an axis and that receives an elongate pin to connect eachfitting to the structure.
 5. The strut of claim 3, wherein: each fittingincludes a plurality of outer steps; and the laminated composite tubeincludes groups of plies respectively overlapping the outer steps,wherein the outer steps and the groups of plies form a single steppedshear joint.
 6. The strut of claim 3, wherein each fitting is metal andis attached to the laminated composite tube by a single shear joint. 7.The strut of claim 4, wherein each stiffener of the pair of longitudinalstiffeners is substantially symmetric about a plane perpendicular to theaxis of the opening.
 8. The strut of claim 1, further comprising: asecond pair of stiffeners within the substantially hollow interior ofthe laminated composite tube and attached to the laminated compositetube.
 9. The strut of claim 8, wherein: the second pair of stiffeners issubstantially symmetric about a plane substantially parallel to acentral axis of an opening of a fitting on one end of the laminatedcomposite tube that attaches to a structure; and each of the second pairof stiffeners is trapezoidal in cross sectional shape.
 10. The strut ofclaim 1, wherein each of the pair of longitudinal stiffeners is alaminated composite and is cocured with the laminated composite tube.11. A strut for transferring loads in compression or tension,comprising: an elongate tube having a wall formed of multiple laminatedcomposite plies; a fitting on each end of the elongate tube thatattaches the elongate tube to a structure; a single shear bond jointbetween each fitting and the elongate tube; and composite stiffenersattached to the wall of the elongate tube for stiffening the elongatetube.
 12. The strut of claim 11, wherein: each fitting attaches theelongate tube to the structure with a pin and transfers loads to thetube about a longitudinal axis of each pin; and the composite stiffenersare located inside the elongate tube and include a first pair ofopposing stiffeners aligned along an axis substantially perpendicular tothe longitudinal axis of each pin.
 13. The strut of claim 12, whereinthe composite stiffeners include a second pair of opposing stiffenersaligned along an axis extending substantially parallel to thelongitudinal axis of each pin.
 14. The strut of claim 11, wherein: eachfitting includes an outer wall having a plurality of outer steps; andthe elongate tube includes groups of plies respectively overlapping theouter steps and forming a single stepped shear joint.
 15. The strut ofclaim 11, wherein each of the composite stiffeners is a laminatedcomposite and is cocured with the elongate tube.
 16. The strut of claim11, wherein each of the composite stiffeners extends substantially anentire length between each fitting.
 17. A method of making a strut,comprising: locating composite stiffener layups on an elongate mandrel;forming a composite tube by laying up composite plies on the elongatemandrel overlying the composite stiffener layups; cocuring the tube andthe composite stiffener layups; and removing the elongate mandrel. 18.The method of claim 17, further comprising: fabricating the elongatemandrel.
 19. The method of claim 17, further comprising: placing afitting on each end of the elongate mandrel.
 20. The method of claim 17,further comprising: forming an axle on each end of the elongate mandrel;forming cavities in the mandrel along its length; and placing arespective fitting on each end of the elongate mandrel.
 21. The methodof claim 20, wherein placing the respective fitting on each end of theelongate mandrel comprises placing the respective fitting on arespective axle on each end of the elongate mandrel.
 22. The method ofclaim 17, wherein locating the stiffener layups on the elongate mandrelincludes: laying up and forming the stiffener layups to create formedstiffener layups; and placing the formed stiffener layups in cavities inthe elongate mandrel.
 23. The method of claim 17, wherein forming thecomposite tube is performed using an automated fiber placement machineto laydown and compact strips of composite tape on a surface of theelongate mandrel.
 24. The method of claim 17, wherein locating thestiffener layups on the elongate mandrel includes using an automatedfiber placement machine to laydown and compact strips of composite tapewithin cavities of the elongate mandrel.
 25. The method of claim 19,wherein forming the composite tube includes forming a single shear jointbetween the composite tube and each fitting.
 26. The method of claim 17,wherein removing the elongate mandrel includes dissolving the elongatemandrel with a liquid.
 27. The method of claim 17, further comprising:inserting stiffener mandrels within the composite stiffener layups; andremoving the stiffener mandrels after the composite tube and thecomposite stiffeners have been cocured.
 28. A method of making a strutfor aircraft applications, comprising: fabricating an elongate mandrel,including forming cavities in the elongate mandrel along its length andforming axles on each end of the elongate mandrel; sleeving metal endfittings over the axles of the elongate mandrel; locating a plurality ofcomposite stiffener layups in the cavities of the elongate mandrel, eachcomposite stiffener of the plurality of composite stiffener layupshaving a cross sectional shape corresponding to the cross sectionalshape of the cavities of the elongate mandrel; placing stiffenermandrels in the plurality of composite stiffener layups; forming acomposite tube by laying up composite plies on the elongate mandreloverlying the plurality of composite stiffener layups, the metal endfittings, and the stiffener mandrels; cocuring the tube and theplurality of composite stiffener layups; and removing the stiffenermandrels and the elongate mandrel by dissolving the stiffener mandrelsand the elongate mandrel.
 29. An aircraft strut for transferring loadsin compression or tension, comprising: an elongate tube having a wallformed of multiple laminated composite plies; a fitting on each end ofthe tube adapted to attach the elongate tube to a structure with a pin;a single stepped shear bond joint between each of the fittings and theelongate tube; and composite stiffeners within the elongate tubeattached to the wall for stiffening the elongate tube, and wherein: eachfitting transfers loads to the elongate tube about a longitudinal axisof the pin, and the composite stiffeners include at least a first pairof opposing stiffeners aligned along an axis substantially perpendicularto the axis of the pin; each of the composite stiffeners has asubstantially trapezoidal cross sectional shape; the compositestiffeners further include a second pair of opposing stiffeners alignedalong an axis extending substantially parallel to the pin axe; eachfitting includes an outer wall having a plurality of outer steps, and asubstantially smooth inner wall; and the elongate tube includes groupsof plies respectively overlapping the outer steps.
 30. An apparatuscomprising: a mandrel comprising a generally cylindrical body and anumber of circumferentially spaced, longitudinal cavities; stiffenerlayups within each of the cavities; stiffener mandrels inside thestiffener layups; and composite plies laid up over the mandrel.
 31. Theapparatus of claim 30, wherein the stiffener layups comprise strips offiber reinforced prepreg tape.
 32. The apparatus of claim 30, whereinthe cavities are trapezoidal in cross-sectional shape.
 33. The apparatusof claim 30, wherein the mandrel further comprises two generallycylindrical axles and further comprising: metal end fittings sleevedover the two generally cylindrical axles, wherein the composite pliesare laid up over portions of the metal end fittings.
 34. An apparatuscomprising: a mandrel comprising a generally cylindrical body, twogenerally cylindrical axles, and a number of circumferentially spaced,longitudinal cavities; and metal end fittings sleeved over the twogenerally cylindrical axles.
 35. The apparatus of claim 34, wherein themandrel is formed of a material which may be dissolved away whensubjected to a suitable liquid.
 36. The apparatus of claim 34, whereinthe mandrel is formed of a material that is incinerated when elevated toa predetermined temperature.
 37. The apparatus of claim 34, wherein themandrel comprises an inflatable bladder.
 38. The apparatus of claim 34,further comprising: stiffener mandrels within the cavities, eachstiffener mandrel comprising either a dissolvable solid material or aninflatable bladder.